MUMBAI/BENGALURU (Reuters) - Jet Airways shares plunged on Thursday, battered by worries about whether the grounded Indian airline would fly again, while carriers such as SpiceJet rushed in to plug a supply gap in the wake of their rival's demise.Once India's largest private airline, Jet halted all flight operations indefinitely on Wednesday evening after lenders led by the State Bank of India declined to extend more funds to keep the carrier going. Investors punished Jet shares as trading..